1. Technical Field
This invention relates to a method of thin plating steel with chromium; more particularly, it relates to the use of an electrolyte of a particular ionic form to make "tin-free steel."
2. Description of the Related Art
In the American steel industry, low carbon sheet steel thinly plated with chromium is generally referred to as tin free steel ("TFS"). In Europe and Asia, the same type of product is more commonly called electrolytically chromium coated steel ("ECCS"). In either case, the typical practice is to prepare an electrolyte containing 70 to 120 grams per liter of CrO.sub.3, (chromic acid) together with small amounts of sulfate ions (about 0.2-0.8 grams per liter) and fluoride ions (about 1.0 to 5.0 grams per liter). This invention does not concern the making of decorative and hard chromium plates, which utilizes baths containing generally 200 to 400 g/L.
In the manufacture of TFS, the sheet steel is utilized as the cathode in an electrolytic cell for the solution, power is applied, and a coating of chromium metal is formed on the sheet steel. Typically, the finished product will have a layer of 3 to 13 milligrams per square foot of metallic chromium (a common target is 5 mg/ft.sup.2) and will have a layer of about 0.5 to 1.5 mg/ft.sup.2 chromium oxide on top of the metallic chromium. The strip must be rinsed quickly and properly to avoid significant staining. Large quantities of this product are made by continuous strip plating.
The conventional electrolytic chrome treatment as described above presents formidable environmental problems and expense. One response to these problems and expense is to search for a way to reduce the amount of waste electrolyte generated. The literature, however, discourses an approach of reducing the quantity of chromic acid in the electrolyte. See, for example, lines 2-32, of column 4 of Allen et al U.S. Pat. No. 3,642,587: "Forty g/l of CrO.sub.3 has been found to be a minimum amount useful in the baths contemplated herein because below that amount bright chromium plate cannot be obtained by the process of the invention, and, further, a heavy, dark-colored coating of hydrated chromium oxide is produced." The baths used by Allen et al included sulfate and fluoride catalysts.
A study by Marcel Pourbaix, "Atlas of Electrochemical in Equilibria in Aqueous Solutions," National Association of Corrosion Engineers, Houston (Library of Congress 65-11670), Second English Edition 1974, pages 256-271, illustrates the complexity of the chromate ion and its various states in aqueous solution. Numerous forms of chromate ions are shown. But, although the term "hexavalent chromium" is commonly used, a simple Cr.sup.+6 ion has never been identified. When chromium trioxide (CrO.sub.3) dissolves in water, it forms chromic acid: CrO.sub.3 +H.sub.2 O.fwdarw.H.sub.2 CrO.sub.4 (Equation 1). The chromic acid is at equilibrium with HCrO.sub.4.sup.- (acid chromate ion) and Cr.sub.2 O.sub.7.sup.= (dichromate ion) as shown in H.sub.2 CrO.sub.4 &lt;--&gt; HCrO.sub.4.sup.- +H.sup.+ (Equation 2) and 2H.sub.2 CrO.sub.4.sup.= &lt;- - &gt; Cr.sub.2 O.sub.7.sup.= +H.sub.2 O+2H.sup.+ (Equation 3). Three types of hexavalent anion are generated: HCrO.sub.4.sup.-, CrO.sub.4.sup.=, and Cr.sub.2 O.sub.7.sup.= ; their concentrations in the solution depend on the pH and the initial CrO.sub.3 concentration. It is known, as pointed out in "Industrial Electrochemical Processes" edited by A. T. Kuhn, Elsevier Publishing Company (1971) page 354, that "(T)he [HCrO.sub.4 ].sup.- ion predominates in dilute CrO.sub.3 solutions, whereas the [HCr.sub.2 O.sub.7 ].sup.- ion is formed preferentially in concentrated solutions, such as those that are used for plating baths." The [HCr.sub.2 O.sub.7 ].sup.- ion may also exist as Cr.sub.2 O.sub.7.sup.=. This publication goes on to discuss the importance of the presence of trivalent chromium to the deposition of the chromium layer, particularly focusing on the role of the sulfuric acid catalyst in preventing the formation of an impermeable film primarily of Cr(OH)CrO.sub.4.
It is desirable to reduce the environmental consequences in the manufacture of tin free steel in spite of the complexities presented by chromate electrochemistry.